fix: enforce string type validation in string_plus and remove coercion TODO

- Replace silent null return with explicit string type check in string_plus native
- Remove outdated TODO comments about string coercion from both num_plus and string_plus
- Add test for basic string concatenation ("a" + "b")
- Add test verifying runtime error when concatenating string with non-string ("a" + 123)

test/string/concatenation.wren

@@ -0,0 +1 @@
+IO.print("a" + "b") // expect: ab

test/string/concatenation_wrong_arg_type.wren

@@ -0,0 +1 @@
+IO.print("a" + 123) // expect runtime error: Right operand must be a string.